Method and apparatus providing a multi-function terminal for a power supply controller

Abstract

A power supply controller having a multi-function terminal. In one embodiment, a power supply controller for switched mode power supply includes a drain terminal, a source terminal, a control terminal and a multi-function terminal. The multi-function terminal may be configured in a plurality of ways providing any one or some of a plurality of functions including on/off control, external current limit adjustments, under-voltage detection, over-voltage detection and maximum duty cycle adjustment. The operation of the multi-function terminal varies depending on whether a positive or negative current flows through the multi-function terminal. A short-circuit to ground from the multi-function terminal enables the power supply controller. A short-circuit to a supply voltage from the multi-function terminal disables the power supply controller. The current limit of an internal power switch of the power supply controller may be adjusted by externally setting a negative current from the multi-function terminal. The multi-function terminal may also be coupled to the input DC line voltage of the power supply through a resistance to detect an under-voltage condition, an over-voltage condition and/or adjust the maximum duty cycle of power supply controller. Synchronization of the oscillator of the power supply controller may also be realized by switching the multi-function terminal to power or ground at the desired times.

What is claimed is:  
     
       1. A power supply controller circuit, comprising: 
       a current input circuit coupled to receive a current representative of an input voltage applied to a power supply, the current input circuit to generate a maximum duty cycle adjustment signal in response thereto; and  
       a control circuit to generate a switching waveform, the control circuit coupled to receive the maximum duty cycle adjustment signal, the control circuit to limit the duty cycle of the switching waveform to a maximum value in response to the maximum duty cycle adjustment signal, the switching waveform to regulate the power supply output.  
     
     
       2. The power supply controller circuit of  claim 1  further comprising a power switch coupled to a primary winding of the power supply, the power switch coupled to receive and to switch in response to the switching waveform. 
     
     
       3. The power supply controller circuit of  claim 2  wherein the maximum duty cycle adjustment signal is inversely adjusted by the current representative of the voltage input to the power supply when the current representative of the voltage input to the power supply is greater than a first value. 
     
     
       4. The power supply controller circuit of  claim 3  wherein the maximum duty cycle adjustment signal is independent of the current representative of the voltage input to the power supply when the current representative of the voltage input to the power supply is less than the first value. 
     
     
       5. The power supply controller circuit of  claim 1  wherein the maximum duty cycle adjustment signal is combined with a control signal received by the control circuit. 
     
     
       6. The power supply controller circuit of  claim 5  wherein the control circuit is a pulse width modulation circuit that includes a comparator to compare an oscillating sawtooth waveform with the combined maximum duty cycle adjustment signal and the control signal received by the pulse width modulation circuit. 
     
     
       7. The power supply controller circuit of  claim 6  wherein the duty cycle of the switching waveform is adjusted in response to an output of the comparator. 
     
     
       8. A method for controlling a power supply, comprising: 
       receiving a first current representative of an input voltage to the power supply through a first terminal of a power supply controller;  
       switching a second current flowing through a primary winding with a switching waveform having a duty cycle;  
       adjusting the duty cycle of the switching waveform in response to the first current.  
     
     
       9. The method of  claim 8  wherein the limit to the duty cycle is reduced in response to an increase in the first current if the first current is greater than a first threshold value, the first threshold having a hysteresis of greater than or equal to zero. 
     
     
       10. The method of  claim 9  further comprising leaving unchanged the duty cycle of the switching waveform if the first current is less than or equal to the first threshold value. 
     
     
       11. The method of  claim 8  wherein adjusting the maximum duty cycle of the switching waveform comprises: 
       generating a positive current sense signal in response to the first current;  
       generating a first voltage in response to the positive current sense signal;  
       comparing the first voltage with a switching sawtooth waveform; and  
       resetting a latch to limit the maximum duty cycle of the switching waveform in response to comparing the first voltage with the switching sawtooth waveform.  
     
     
       12. The method of  claim 8  wherein receiving the first current representative of the input voltage to the power supply comprises coupling a resistance between the first terminal and an input of the primary winding.